client/systray: support several different color themes

Currently we only have a dark theme icon with white and grey dots over
a black background. For some desktops, a logo with black and grey dots
over a white background might be preferable. And for desktops where the
bar is *almost* black or white, but not quite, an option to render the
logo with dots only and no background can look really nice.

Add a new -theme flag to the systray command with the default staying
the same as it is today.

Updates #18303

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

cmd/systray/systray.go

@@ -15,9 +15,11 @@ import (
 )
 
 var socket = flag.String("socket", paths.DefaultTailscaledSocket(), "path to tailscaled socket")
+var theme = flag.String("theme", "dark", "color theme for Tailscale icon: dark, dark:nobg, light, light:nobg")
 
 func main() {
 	flag.Parse()
 	lc := &local.Client{Socket: *socket}
+	systray.SetTheme(*theme)
 	new(systray.Menu).Run(lc)
 }